Recently, with the advances society has made in technology and the expansion of information and communication equipment, computation equipment, online service equipment, automated production line and precise control equipment, there has been a lot of research into superconducting magnetic energy storage (SMESs) aiming to provide high-quality power to sensitive and important loads placed on equipment. There is a variety of superconducting magnetic energy storage, including small superconducting magnetic energy storage which is used to control the quality of power, and large superconducting magnetic energy storage which are used to equalize a load. Recently, small superconducting magnetic energy storage in several MJ class for purposes of controlling the power quality of sensitive loads has been commercialized for industrial and military use, and their effect has been proven.
Such a superconducting magnetic energy storage includes a superconducting magnet comprising some superconducting coils, a cryostat which contains the superconducting magnet, a pair of current leads which leads two terminals of the superconducting magnet to the outside of the cryostat, and a power converter which supplies power from an electric power system after converting the power.
In the conventional art, a thin tape-shaped superconducting coil wire is wound up in a pancake shape to form a superconducting coil. Two superconducting coils are used in pair in a double pancake shape. A superconducting magnet is formed by placing double-pancake-shaped superconducting coils one on top of another. In the case of the superconducting coil, depending on the magnitude of a vertical magnetic field perpendicular to a surface, in detail, a large surface, of the pancake-shaped superconducting coil, the characteristics of the critical current become vastly different. As the magnitude of the vertical magnetic field increases, the critical current is reduced, resulting in the problem of the operating current of the superconducting magnet eventually being reduced.
In an effort to overcome the above problem, a method was proposed, in which, instead of being placed one on top of another, the superconducting coils are arranged in a toroidal structure to reduce the vertical magnetic field of the superconducting coils when storing energy in the superconducting magnet.
However, in this conventional method, because adjacent superconducting coils form a double-pancake shape in which they are attached parallel to each other, if these superconducting coils are arranged in a toroidal structure, the area of conductive portions that are displaced from the outermost circumferential surface of the toroidal structure is increased. That is, there is the problem of an increase in the vertical magnetic field on the conductive portions that are displaced from the curved surface of the toroidal structure.
Particularly, in the case of a superconducting coil wire with a width of 4 mm which is widely used, the effect of the toroidal structure that reduces the magnitude of the vertical magnetic field is markedly reduced, because the area of the conductive portions that are displaced from the outermost circumferential surface of the toroidal structure increases, as the width of the superconducting coil wire increases.